The present invention relates to fluids useful in subterranean operations, and more particularly, to viscoelastic surfactant fluids, and methods of using such fluids and controlling the viscosity thereof.
Viscosified treatment fluids may be used in a variety of subterranean treatments. Such treatments include, but are not limited to, drilling operations, stimulation treatments, and sand control treatments. As used herein, the term “treatment,” or “treating,” refers to any subterranean operation that uses a fluid in conjunction with a desired function and/or for a desired purpose. The term “treatment,” or “treating,” does not imply any particular action by the fluid.
An example of one such subterranean treatment is a drilling operation, wherein a treatment fluid (e.g., a drilling fluid) passes down through the inside of the drill string, exits through the drill bit, and returns to the drilling rig through the annulus between the drill string and well bore. The circulating drilling fluid, among other things, lubricates the drill bit, transports drill cuttings to the surface, and balances the formation pressure exerted on the well bore. Drilling fluids typically require sufficient viscosity to suspend drill cuttings. Viscosified treatment fluids also may be used in other operations to transport and remove formation particulates from the well bore or the near well bore region. In some instances, these formation particulates may be generated during the course of drilling, digging, blasting, dredging, tunneling, and the like in the subterranean formation.
A common production stimulation operation that employs a viscosified treatment fluid is hydraulic fracturing. Hydraulic fracturing operations generally involve pumping a treatment fluid (e.g., a fracturing fluid) into a well bore that penetrates a subterranean formation at a sufficient hydraulic pressure to create or enhance one or more cracks, or “fractures,” in the subterranean formation. The fracturing fluid may comprise particulates, often referred to as “proppant particulates,” that are deposited in the fractures. The proppant particulates function, inter alia, to prevent the fractures from fully closing upon the release of hydraulic pressure, forming conductive channels through which fluids may flow to the well bore. Once at least one fracture is created and the proppant particulates are substantially in place, the viscosity of the fracturing fluid usually is reduced (i.e., “breaking” the fluid), and the fracturing fluid may be recovered from the formation. The term “break” and its derivatives, as used herein, refer to a reduction in the viscosity of a fluid, e.g., by the breaking or reversing of the crosslinks between polymer molecules in the fluid, or breaking chemical bonds of gelling agent polymers in the fluid. No particular mechanism is implied by the term.
Another production stimulation operation that employs a viscosified treatment fluid is an acidizing treatment. In acidizing treatments, subterranean formations comprising acid-soluble components, such as those present in carbonate and sandstone formations, are contacted with a treatment fluid comprising an acid. For example, where hydrochloric acid contacts and reacts with calcium carbonate in a formation, the calcium carbonate is consumed to produce water, carbon dioxide, and calcium chloride. After acidization is completed, the water and salts dissolved therein may be recovered by producing them to the surface, e.g., “flowing back” the well, leaving a desirable amount of voids (e.g., wormholes) within the formation, which enhance the formation's permeability and may increase the rate at which hydrocarbons may subsequently be produced from the formation.
Viscosified treatment fluids are also utilized in sand control treatments, such as gravel-packing treatments, wherein a treatment fluid, which is usually viscosified, suspends particulates (commonly referred to as “gravel particulates”) for delivery to a desired area in a well bore, e.g., near unconsolidated or weakly consolidated formation zones, to form a gravel pack to enhance sand control. One common type of gravel-packing operation involves placing a sand control screen in the well bore and packing the annulus between the screen and the well bore with the gravel particulates of a specific size designed to prevent the passage of formation sand. The gravel particulates act, inter alia, to prevent the formation particulates from occluding the screen or migrating with the produced hydrocarbons, and the screen acts, inter alia, to prevent the particulates from entering the production tubing. Once the gravel pack is substantially in place, the viscosity of the treatment fluid is often reduced to allow it to be recovered. In some situations, fracturing and gravel-packing treatments are combined into a single treatment (commonly referred to as “frac pack” operations) to provide stimulated production and an annular gravel pack to reduce formation sand production.
In a variety of subterranean operations, it also may be desirable to divert treatment fluids in a subterranean formation because, among other reasons, the treatment fluid may enter portions of a subterranean formation with high permeability preferentially at the expense of portions of the subterranean formation with lesser permeability. For example, in acid stimulation operations, it may be desired to contact less permeable portions of the subterranean formation with a treatment fluid containing an acid so as to achieve the desired stimulation. Certain diverting techniques involve the placement of viscosified fluids in a subterranean formation so as to plug off the high-permeability portions of the formation, thereby diverting subsequently injected fluids to less permeable portions of the formation. In certain techniques, a treatment fluid is placed adjacent to a certain portion of a subterranean formation, and the treatment fluid is viscosified so as to form a gel that, inter alia, temporarily plugs the perforations or natural fractures in that portion of the formation. The term “gel,” as used herein, and its derivatives include semi-solid, jelly-like states assumed by some colloidal dispersions. When another treatment fluid encounters the gel, it may be diverted to other portions of the formation.
Maintaining sufficient viscosity in treatment fluids may be important for a number of reasons. Viscosity is desirable in drilling operations since treatment fluids with higher viscosity can, among other things, transport solids, such as drill cuttings, more readily. Maintaining sufficient viscosity is important in fracturing treatments for particulate transport, as well as to create or enhance fracture width. Particulate transport is also important in sand control treatments, such as gravel packing. Maintaining sufficient viscosity may be important to control and/or reduce leak-off into the formation, improve the ability to divert another fluid in the formation, and/or reduce pumping requirements by reducing friction in the well bore. At the same time, while maintaining sufficient viscosity of a treatment fluid often is desirable, it also may be desirable to maintain the viscosity of the treatment fluid in such a way that the viscosity may be reduced at a particular time, inter alia, for subsequent recovery of the fluid from the formation.
To provide the desired viscosity, polymeric gelling agents commonly are added to the treatment fluids. The term “gelling agent” is defined herein to include any substance that is capable of increasing the viscosity of a fluid, for example, by forming a gel. Examples of commonly used polymeric gelling agents include, but are not limited to guar gums and derivatives thereof, cellulose derivatives, biopolymers, and the like. The use of polymeric gelling agents, however, may be problematic. For instance, these polymeric gelling agents may leave an undesirable gel residue in the subterranean formation after use, which can impact permeability. As a result, costly remedial operations may be required to clean up the fracture face and proppant pack. Foamed treatment fluids and emulsion-based treatment fluids have been employed to minimize residual damage, but increased expense and complexity often have resulted.
To combat perceived problems associated with polymeric gelling agents, some surfactants have been used as gelling agents. It is well understood that, when mixed with a fluid in a concentration above the critical micelle concentration, the molecules (or ions) of surfactants may associate to form micelles. The term “micelle” is defined to include any structure that minimizes the contact between the lyophobic (“solvent-repelling”) portion of a surfactant molecule and the solvent, for example, by aggregating the surfactant molecules into structures such as spheres, cylinders, or sheets, wherein the lyophobic portions are on the interior of the aggregate structure and the lyophilic (“solvent-attracting”) portions are on the exterior of the structure. These micelles may function, among other purposes, to stabilize emulsions, break emulsions, stabilize a foam, change the wettability of a surface, solubilize certain materials, and/or reduce surface tension. When used as a gelling agent, the molecules (or ions) of the surfactants used associate to form micelles of a certain micellar structure (e.g., rodlike, wormlike, vesicles, etc., which are referred to herein as “viscosifying micelles”) that, under certain conditions (e.g., concentration, ionic strength of the fluid, etc.) are capable of, inter alia, imparting increased viscosity to a particular fluid and/or forming a gel. Certain viscosifying micelles may impart increased viscosity to a fluid such that the fluid exhibits viscoelastic behavior (e.g., shear thinning properties) due, at least in part, to the association of the surfactant molecules contained therein. As used herein, the term “viscoelastic surfactant fluid” refers to fluids that exhibit or are capable of exhibiting viscoelastic behavior due, at least in part, to the association of surfactant molecules contained therein to form viscosifying micelles. Moreover, because the viscosifying micelles may be sensitive to hydrocarbons, the viscosity of these viscoelastic surfactant fluids may be reduced after introduction into the subterranean formation without the need for certain types of gel breakers (e.g., oxidizers). The term “breaker” is defined herein to include any substance that is capable of decreasing the viscosity of a fluid. This may allow a substantial portion of the viscoelastic surfactant fluids to be produced back from the formation without the need for expensive remedial treatments.
However, the use of surfactants as gelling agents may be problematic in several respects. First, high concentrations (i.e., above about 2,000 parts per million (“ppm”)) of certain metal ions such as iron may make the use of surfactants as gelling agents impractical because such metal ions may interfere with the ability of the surfactant molecules to form viscosifying micelles. Lower concentrations (i.e., above about 800 ppm) of certain metal ions also may interfere with the ability of the surfactant molecules to form viscosifying micelles. Moreover, conventional methods of reducing the viscosity of a viscoelastic surfactant fluid generally rely on formation fluids and/or the addition of hydrocarbons (e.g., in an oil-emulsified acid) to provide the conditions needed to reduce the viscosity of the fluid, each of which may be undesirable for any number of reasons. Reliance on formation fluids to provide the conditions needed to reduce the viscosity of the fluid may delay the progress of a particular subterranean operation. Altering the pH of the viscoelastic surfactant fluid may, inter alia, require the addition of additives that increase the cost and/or complexity of the fluid, cause damage to certain substances present within the formation (e.g., minerals), and/or create pH conditions that are undesirable for subsequent operations to be performed. Finally, the addition of an oil-phase additive to an aqueous treatment fluid may, inter alia, decrease the relative permeability of a formation, causing it to become oil-wet, or forming an emulsion, any of which may hinder the production of fluids from the formation.